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INTRODUCTION

Viral infections are among the most common illnesses encountered in the ED. Although many are self-limited, some are life-threatening, have specific treatments, or have public health implications. We review serious viral infections that cause disseminated illness or have a predilection for the CNS.

INFLUENZA

Human influenza is caused by a large, single-stranded RNA virus from the orthomyxovirus family. Influenza A and Influenza B cause the majority of human infections, with influenza A typically being more serious. Other viruses can cause similar febrile respiratory syndromes, including the coronaviruses linked to severe acute respiratory syndrome and Middle Eastern respiratory syndrome, both emanating from limited geographic areas and associated with high mortality.

EPIDEMIOLOGY

Influenza is highly infectious and is transmitted via aerosolized respiratory secretions, large droplets, or fomites. Seasonal influenza has varying severity, typically striking in the winter months of temperate climates. Seasonal disease causes 250,000 to 500,000 annual deaths worldwide and between 5000 and 50,000 deaths in the United States. Outbreaks spread quickly throughout a community, largely among school-age children. Mortality occurs mostly among the elderly and young infants.

New strains occasionally emerge that can lead to pandemics. The 1918 influenza pandemic killed between 50 and 100 million people worldwide, and the potential for devastation by a novel influenza virus drives anxiety among clinicians and public health personnel. Unlike seasonal influenza, mortality during pandemics occurs mostly among healthy adults. In 2009, a pandemic H1N1 strain emerged in Mexico and followed this pattern, reminding us about the potential danger. A highly pathogenic avian strain of influenza emerged in 1997 in China, which was transmitted from infected chickens to humans. A variety of other animal strains of influenza have since been detected. These remain primarily bird viruses that have not developed the ability for efficient human-to-human transmission.

PATHOPHYSIOLOGY

Influenza is transmitted by respiratory secretions, usually by sneezing or coughing, and via touching hands. The incubation period is typically 1 to 4 days. Influenza viruses undergo minor variations (antigenic drift) in their surface antigens that allow the virus to reinfect individuals and reemerge each winter. Occasionally, there are major antigenic changes (antigenic shift) that increase population susceptibility and lead to a pandemic.

CLINICAL FEATURES

Patients with influenza typically present with an abrupt onset of fevers and respiratory symptoms. Most cases are self-limited; however, some patients, particularly the elderly, the very young, and those with comorbid conditions, require hospitalization or die from complications of influenza. Additionally, influenza can lead to exacerbations of chronic medical conditions, such as congestive heart failure or chronic obstructive lung disease. Delirium is common in the elderly.